(1) Field of the Invention
The present invention is directed to a system for introducing tangential control fluid flows along the surface of an aerodynamic member.
(2) Description of the Prior Art
The fluid flow or flow field around lifting surfaces, such as aircraft wings and helicopter rotor blades, determines the pressure distribution over these surfaces and, therefore, determines the resulting aerodynamic forces and moments acting upon such surfaces. Modification and control of those forces is achieved through modification and control of the flow field.
In fixed wing applications, the common method for controlling the flow field involves the use of a movable trailing edge surface (i.e. a wing flap). Moving the trailing edge surface or flap downward increases both the camber and the angle of attack of the fixed wing, increasing the negative pressure distribution along the upper surface of the fixed wing. This increase in negative pressure increases lift.
The traditional use of mechanical flaps, however, requires the use of a hydraulic actuator or actuators, increasing both weight and mechanical complexity. In addition, these mechanical systems have a relatively long response time, inhibiting rapid movement of the trailing edge surface. These limitations prohibit the use of movable trailing edge surfaces in applications where the wing moves and is not fixed (i.e. in helicopter rotor blades). In the case of helicopter rotor blades, the rate of rotation of the rotor blade is several rotations per second or more, and mechanically actuated movable trailing edges cannot respond rapidly enough.
The limitations associated with movable trailing edges in both fixed and rotating wing applications led to the development of alternate methods for controlling the fluid flow around aerodynamic members. One such development involves the blowing of air tangentially along a surface of the wing from one or more slots disposed along that surface. This tangential discharge of gas energizes the fluid flow in the vicinity of the surface of the wing (i.e the boundary layer), to inhibit flow separation and the associated adverse stalling effects. In addition, the lift generated by the wing is increased. An increase in lift is realized from the Coanda effect.
The use of the Coanda effect increases the circulation about an aerodynamic control surface above the level that would otherwise be attainable. This is accomplished by using an airfoil shape with a rounded trailing edge. Fluid injected tangent to the surface in the downstream direction through slots on one side of the airfoil near the trailing edge tends to flow around that edge: the circulation about (and therefore the lift produced by) the airfoil is increased dramatically. This effect was first observed by Henri Coanda in 1910.
Employing the Coanda effect on control surfaces attached to a vehicle has a number of potential benefits, including reduced overall drag and improved maneuverability. These benefits are due to the lift coefficients that are achievable with the Coanda effect. Whereas standard control surfaces, using angle-of-attack to achieve lift, may yield maximum sectional lift coefficients in the range of 0.8-1.0, control surfaces employing the Coanda effect may achieve lift coefficients in the range of 2.0-3.0 or greater.
For example, the trailing edge of the wing is formed as a smoothly curved surface, and the air discharged from the slots follows the smoothly curved surface until the balance between the pressure variation normal to the surface of the wing and the centrifugal force exerted on the discharge of air is lost. The end result is a modification of the pressure distribution along the surface of the wing that is equivalent to the changes created by a mechanical flap. Therefore, the discharge of air along the surface of a wing is an alternative to a movable trailing edge.
Mechanical flaps and the resulting changes in lift are controlled by extending and retracting the flap. When air flow is used, control is achieved by varying the flow rate of the air or the direction in which the air is being discharged, which is known as vectoring. Typically, both variable flow rates and vectoring are used as control mechanisms. Therefore, systems and methods were developed to provide the necessary control over flow rate and vectoring, in particular under real time flight conditions or in rotor blade applications.
A typical system used to control the air flow included a chamber filled with pressurized air. The pressurized air was discharged through a slot disposed adjacent or near the trailing edge of the wing such that the discharged air would travel tangent to the surface of the wing and around the smooth trailing edge. The flow of air out of the slots could be controlled, for example, using a screw mechanism to adjust the size of the slot opening. Another controlling factor of air flow is the pressure of the compressed air inside the chamber. Therefore, the flow rate of the compressed air out of the slot opening increases with increasing pressure in the chamber and decreases with decreasing pressure in the chamber.
The systems used to modify the air flow rate still had a relatively slow response time, especially as the size of the system increased. For example, modification of the flow about a wing requires a change in the size of the slot opening. When the slot opening is increased, the pressure inside the chamber needs to be increased, which requires a substantial amount of time. Conversely, reducing the slot size requires a reduction of the pressure inside the chamber. Pressure reduction is accomplished by bleeding pressure from the chamber, which again takes a significant amount of time to accomplish. Another complicating factor involves the inaccuracies involved in measuring pressure within the chamber.
Other attempts to control the air flow have used camming systems to alter the size of the slot opening mechanically. Such systems, however, are too bulky for many applications and do not significantly improve the response time of the system. Additional limitations of systems used to control the flow of air around the wing surface include the inability to control the airflow independently at different points along the slot or along the length of the wing. Therefore, the air flow out of the slot cannot be varied as a function of position along the wing.
U.S. Pat. Nos. 5,791,601 and 6,142,425 are directed to an apparatus and methods for aerodynamic blowing control using smart material actuators. A source of compressed air is provided that communicates with one or more slots or other outlets at a selected location on an aerodynamic member so that air can be blown from the slots or outlets. The characteristics of the blowing, including activation and deactivation, the magnitude, and the direction of the blown jet are controlled by a valve or nozzle mechanism adapted to interrupt or otherwise affect the compressed air stream provided to the slot. The valve or nozzle, in turn, is activated and controlled through the use of a smart material actuator such as, for example, a piezoelectric bender. In general, the valve or nozzle is formed by aligning corresponding slots or holes in two concentric cylinders or spheres. Therefore, maintaining proper alignment between the corresponding holes is important for providing the desired or calibrated control of the air flow over the aerodynamic member. In addition, the concentric members create a possible source of energy dissipating friction that could result in system failure. The method by which fluid is injected uses two concentric cylinders or spheres rotated relative to one another by smart materials.
For full control, a control surface needs to be able to produce forces in opposite directions. To produce these opposite forces using the Coanda effect, the control surface would have ejection slots on both sides of the control surface and would control the amount of air being ejected through the slots on each side. Prior solutions use separate plumbing and valving for each side or even each slot. This adds complexity to the design and places restrictive lower limits on the size of control surfaces.
Therefore, the need exists for a system to control the flow rate and vectoring of fluids ejected along an aerodynamic member with a response time that is suitable for use with rotating aerodynamic members and that reduces or eliminates friction between moving surfaces. Suitable systems will provide for the variation of flow with both time and location along the length of the aerodynamic member. In addition, full control will be accomplished by producing opposite forces using the Coanda effect.